The present invention is situated in the field of telecommunications, and more particularly in the field of WiFi and 4G/3G/2G mobile networks for SMS (‘Short Message Service’ in English) short message sending and receiving services.
With regard to 2G and 3G access networks, the telephony service and the SMS message sending and receiving service are available in what is called circuit-switched (CS) mode in English.
FIGS. 1A and 1B illustrate the procedures for sending and receiving SMS messages in circuit-switched mode over a 2G/3G cellular access network. These figures schematically show:                A mobile terminal UE;        A mobile switching center V-MSC corresponding to the current location of this terminal (visited Mobile Switching Center in English), ‘V-MSC’ hereinafter;        What is termed a visitor location register VLR in English, ‘VLR’ hereinafter;        What is termed a home location register HLR in English, ‘HLR’ hereinafter;        What is termed a short message service center SMS-C in English, ‘S-MSC’ hereinafter;        An SMS gateway GMSC, ‘gateway GMSC’ hereinafter.        
The sending of an SMS message using a mobile terminal UE (SM-MO ‘Short Message Mobile Originated’ service) is now described with reference to FIG. 1A.
It is assumed in this case that the mobile terminal UE has already attached to the cellular network via the IMSI Attach procedure, and in particular that the VLR has downloaded the profile of the terminal UE from the HLR.
During a step E2, the terminal UE sends the SMS message to the V-MSC, using the command SMS-SUBMIT.
During a step E4, the V-MSC transmits, to the VLR with which it is associated (hereinafter ‘register VLR’), a message MAP-SEND-INFO-FOR-MO-SMS so as to ask it for the MSISDN number of the terminal UE and to verify, from the profile of the user of this terminal, that there is no restriction imposed on this user.
The VLR returns an affirmative acknowledgement MAP-SEND-INFO-FOR-MO-SMS-ACK to the V-MSC during a step E6.
If this acknowledgement is affirmative, the V-MSC sends (step E8) the SMS message to the SMS-C, using the message MAP-MO-FORWARD-SHORT-MESSAGE. This message contains the MISDN number of the terminal UE, the MSISDN number of the recipient entered by the user, the number of the SMS-C stored in the SIM card of the terminal UE, and the characters of the SMS message to be transmitted.
The SMS-C stores the message and the MSISDN numbers of the sender and of the recipient and then sends the SMS message to the recipient during a step E10. It receives an affirmative response of sending of the SMS message (step E12).
During a step E14, the SMS-C includes the transmission report in a message MAP-MO-FORWARD-SHORT-MESSAGE-ACK destined for the V-MSC, and the V-MSC returns a message SMS-STATUS-REPORT to the mobile terminal UE (step E16).
The reception of an SMS message originating from the SMS-C(SM-MT ‘Short Message Mobile Terminated service) by a mobile terminal UE is described with reference to FIG. 1B.
During a step F2, the gateway GMSC receives an SMS message originating from a center SMS-C.
During a step F4, the gateway GMSC asks the HLR for routing information for the SMS message via the request MAP-SEND-ROUTING-INFO-FOR-SMS. This request in particular contains the MSISDN number of the recipient terminal.
During a step F6, the HLR returns, to the gateway GMSC, via the message MAP-SEND-ROUTING-INFO-FOR-SMS-ACK, the routing information corresponding to the address of the V-MSC in charge of the recipient terminal, as well as the IMSI number of this recipient terminal.
During a step F8, the gateway GMSC sends the message MAP-MT-FORWARD-SHORT-MESSAGE to the V-MSC.
During a step F10, the V-MSC transmits a request MAP-SEND-INFO-FOR-MT-SMS to its local VLR for the purpose of obtaining information relating to the recipient, this request including the IMSI number of the recipient terminal.
From the IMSI number, the VLR identifies what is called the location area in English, or LA, of the recipient terminal.
During a step F12, the VLR initiates a paging procedure (MAP-PAGE) consisting in carrying out a search over the entire area where the recipient terminal for the SMS message is liable to be located. If the VLR does not know the identity of the recipient, a message MAP-SEARCH-FOR-SUBSCRIBER is transmitted so as to launch the search procedure over all of the locations LA that are dependent on the V-MSC. In the case of FIG. 1B, the identity of the recipient terminal is assumed to be known to the V-MSC. The paging procedure is initiated by the VLR, but carried out by the V-MSC.
The V-MSC carries out the paging procedure over the location area LA of the recipient during a step F14; the recipient terminal responds affirmatively to this paging procedure during a step E16.
During a step F18, the VLR returns a response MAP-SEND-INFO-FOR-MT-SMS-ACK to the V-MSC, authorizing the latter to relay the SMS message to the recipient terminal.
During a step F20, the V-MSC routes the SMS message to the recipient mobile terminal via the message SMS-DELIVER, and receives an acknowledgement report SMS-STATUS-REPORT.
During a step F22, the V-MSC includes the delivery report of the SMS message to the recipient terminal in the response MAP-MT-FORWARD-SHORT-MESSAGE-ACK returned to the gateway GMSC.
The SMS gateway GMSC transfers this report to the SMS-C during a step F24.
When the mobile terminal UE is connected to a 4G cellular network, and the VoLTE (Voice over Long Term Evolution) service is not available either because the telecommunications operator does not have this service or the subscription of the client does not authorize this service or the terminal of the client does not yet have this functionality, it uses the fallback mode CS FallBack to receive and transmit telephone calls.
In accordance with this mode, the 4G network asks the mobile terminal UE to switch, when such a network is available, to a 2G or 3G network to process the call. Then, at the end of communication, the mobile terminal UE switches back to the 4G network if the latter is available.
This fallback procedure CS Fallback is relatively burdensome and, in any case, poorly suited to sending and receiving SMS messages.
In a known manner and as shown in FIG. 2A, the terminal under 4G coverage asks, in order to send and receive SMS messages, to attach to the network in ‘Combined IMSI Attach’ mode (step K1).
This figure shows:                What is termed a mobility management entity in English, ‘MME’ hereinafter;        What is termed an enhanced mobile switching center in English, MSC-e hereinafter.        
In accordance with the Combined IMSI Attach procedure, when the terminal attaches to the 4G network, it sends an explicit Combined Attach request to its MME attachment device. Upon reception of such a command, the MME identifies the enhanced mobile switching center MSC-e of the area local to the terminal and sends to it a request such that it takes charge of the SMS messages of this terminal.
Upon reception of this command, the MSC-e sends a MAP protocol command to the HLR, such that the latter routes the SMS messages destined for the terminal UE via this MSC-e.
The sending of an SMS message in accordance with the Combined Attach procedure (or SMSoSGs) is described with reference to FIG. 2B. In accordance with this procedure, when the terminal UE wishes to send an SMS message, it encapsulates this SMS message in a message of the NAS (Network Access Stratum) layer destined for the MME (step G2).
The MME transmits this message via the SGs interface to the MSC-e selected during the Combined IMSI attach network attachment phase (step G4), and then the MSC-e sends the SMS message in circuit-switched mode, as if this SMS message had been sent using a 2G/3G cellular radio access.
Steps similar to steps E8 to E14, already described with reference to FIG. 1A, are in particular implemented.
FIG. 2C recalls, in the same way, the known procedure of receiving an SMS message in Combined Attach mode (or SMSoSGs); this is not described in detail here.
The Combined Attach procedure has the advantage of being much more flexible than the fallback procedure CS fallback used to manage telephone calls. In particular, it allows clients attached to 4G to remain on 4G to transmit and receive SMS messages, and therefore does not disrupt client experience by way of a significant throughput reduction, for example when the client is streaming a video.
On the other hand, it requires a development of the V-MSC to support the SGs interface, and imposes an additional load on the V-MSC.
This procedure furthermore has the drawback of not being applicable to terminals that have only WiFi coverage.
It is in particular to rectify this drawback that the 3GPP standardized the mode for sending and receiving SMS messages over an IP network (SMS over IP or ‘SMSoIP’ in English), which mode is able to be used in the packet-switched PS domain, that is to say both under 4G cellular network coverage with VxLTE (Voice-Video-etc over LTE) service support and under WiFi coverage with VxWiFi (Voice-Video-etc over WiFi) service support.
The known operation of SMSoIP mode is recalled with reference to FIGS. 3A to 3C. These figures show a gateway for managing SMS messages over an IP network (Internet Protocol Short Message Gateway in English), ‘gateway IP-SM-GW’ hereinafter.
Once the terminal UE has attached to the 4G cellular network (step G2), the latter registers (step G4) via the SIP protocol in the IMS subsystem, that is to say as far as the S-CSCF.
The S-CSCF extends the SIP registration as far as the gateway IP-SM-GW via the SIP Third Party Registration procedure (step G6).
The SIP registration message transmitted to the gateway IP-SM-GW contains, in the AoR (Address of Record) field, the public identity of the terminal UE, namely its MSISDN number.
This gateway IP-SM-GW has a SIP interface; this may be collocated in the Telephony Application Server application server, in an SMS-C, or in an independent device. Regardless of its implementation, the gateway IP-SM-GW is seen from the IMS core as an application server AS.
After having received the SIP registration message, the gateway IP-SM-GW is optionally able to subscribe (step G8) to the Event Package Reg (for Registration), so as to be informed immediately when the terminal UE is no longer registered in the IMS network core (S-CSCF).
During a step G10, if the contact address AoC supplied by the terminal contains information that it supports the SMS over IP functionality, the gateway IP-SM-GW sends, to the HLR, via a MAP interface, a message MAP ATM (Any Time Modification) request so as to tell it that the SMS messages destined for the terminal UE with the MSISDN number now have to be routed via the gateway IP-SM-GW, and no longer via the V-MSC.
The HLR confirms that the address of the gateway IP-SM-GW has been accepted during a step G12.
The sending of an SMS message in SMSoIP mode is recalled with reference to FIG. 3B.
During a step G14, the SMS message is encapsulated by the terminal UE in a message SIP MESSAGE destined for the S-CSCF (via the P-CSCF, not shown in the figures for the sake of simplicity).
The message SIP MESSAGE is transmitted to the gateway IP-SM-GW (step G16); the gateway IP-SM-GW acknowledges reception of the message SIP MESSAGE by sending a response 202 Accepted to the S-CSCF (step G18). The S-CSCF relays the response 202 Accepted to the terminal UE, via the P-CSCF (step G20).
During a step G22, the gateway IP-SM-GW consults the HLR to verify whether the terminal UE has the rights to send an SMS message.
If sending of an SMS message is authorized, the gateway IP-SM-GW decapsulates the SMS message from the message SIP MESSAGE and re-encapsulates it in a MAP message MAP-MO-FORWARD-SHORT-MESSAGE to the SMS-C (in a manner similar to step E8, already described). Next, the SMS-C sends the SMS message to the network of the recipient of the SMS message, in a manner similar to step E10, already described.
The SMS-C returns the transmission report of sending of the SMS message via the MAP message MAP-MO-FORWARD-SHORT-MESSAGE-ACK to the gateway IP-SM-GW (in a manner similar to step E14, already described).
During a step G24, the gateway IP-SM-GW decapsulates the transmission report for the SMS message from the MAP message and re-encapsulates it in a message SIP MESSAGE and then sends it to the S-CSCF; the S-CSCF sends the message SIP MESSAGE to the terminal UE, via the P-CSCF (step G26); the terminal UE responds 200 OK to the S-CSCF, via the P-CSCF (step G28), so as to acknowledge reception and acceptance of the message SIP MESSAGE; the S-CSCF relays the response 200 OK to the gateway IP-SM-GW (step G30).
The reception of an SMS message in SMSoIP mode is recalled with reference to FIG. 3C.
As described with reference to FIG. 1B, the SMS message is transmitted by the sender to the SMS-C, which may or may not belong to the operator of the recipient, during a step similar to step F2.
During a step similar to step F4, the SMS-C consults the HLR via the message MAP-SEND-ROUTING-INFO-FOR-SMS so as to supply the SMS-C with the routing information for the SMS message (GT ‘Global Title’ address of the gateway IP-SM-GW seen as a VLR).
During a step G32, the HLR relays the message to the gateway IP-SM-GW, the gateway having asked, in step G10, for the SMS messages destined for this MSISDN number to be routed to it.
During a step similar to step F6, already described, the gateway IP-SM-GW returns, via the message MAP-ROUTING-INFO-FOR-SMS-ACK, the GT routing information corresponding to the address of the gateway IP-SM-GW.
The SMS-C then routes the SMS message to the gateway IP-SM-GW via the MAP message MAP-MT-FORWARD-SHORT-MESSAGE in a manner similar to step F8, already described.
The gateway IP-SM-GW decapsulates the SMS message from the MAP message and then re-encapsulates it in a message SIP MESSAGE before sending it to the S-CSCF (step G34).
The S-CSCF transmits the message SIP MESSAGE via the P-CSCF as far as the terminal UE (step G36); the terminal UE acknowledges reception of the message SIP MESSAGE by sending the SIP response 200 OK to the S-CSCF (step G38); the S-CSCF relays the SIP response 200 OK to the gateway IP-SM-GW (step G40).
The terminal UE sends, to the S-CSCF, via the P-CSCF, the delivery report for the SMS message via the message SIP MESSAGE (step G42); the S-CSCF relays the message SIP MESSAGE to the gateway IP-SM-GW (step G44).
During a step G46, the gateway IP-SM-GW acknowledges reception of the message SIP MESSAGE via the response 202 Accepted transmitted to the S-CSCF; the S-CSCF relays, via the P-CSCF, the response 202 Accepted to the terminal UE (step G48).
The gateway IP-SM-GW sends the send report for the SMS message to the SMS-C via the MAP message MAP MAP-MT-FORWARD-SHORT-MESSAGE-ACK during a step similar to step F22, already described.
The sending and reception of SMS messages via IP (SMSoIP) may apply to 4G cellular coverage and to WiFi coverage if the terminal has a VxLTE and/or VxWiFi IMS stack and a suitable subscription to the mobile operator.
It will be noted that, when the terminal UE has the VxWiFi service, the only solution that it has for transmitting or receiving SMS messages is to use SMSoIP mode.
This is why, when the terminal registers in the IMS network core (step G4), the gateway IP-SM-GW is notified, via the Third Party Registration mechanism (step G6), such that this gateway IP-SM-GW gives the instruction (step G10) to the HLR to route the incoming SMS messages for this terminal to this gateway IP-SM-GW.
However, if the terminal UE loses WiFi connectivity and switches to 2G/3G or to 4G (with or without VxLTE), then the path taken to receive the SMS message is not optimal in terms of network resources.
This situation will now be illustrated more precisely with reference to FIGS. 4A to 4C.
It is recalled, with reference to FIG. 4A, that when the terminal UE starts up (ON/OFF button of the terminal or after deactivation of flight mode) under WiFi or 4G cellular coverage, it seeks to register using SIP protocol in the IMS network core in accordance with the mutual authentication procedure (step H2).
During this procedure, the registration messages REGISTER include, in a known manner, a SIP field PANI defining the type of access and one of the parameters of which contains the value eUTRAN for 4G access or WLAN for WiFi access.
At the end of the mutual authentication procedure, when the network responds 200 OK (Expire=1H), the terminal is registered in the IMS network core for a period of 1 hour.
In a known manner, the terminal has to reregister (subsequent registrations) after 50 minutes (=Expires−10 minutes).
Once the terminal UE is registered in the IMS network core and in particular with the S-CSCF, the Third Party Registration function makes it possible to notify the gateway IP-SM-GW so as to tell it that the terminal UE is registered in the IMS network core (step H4).
If the terminal indicates that it supports the SMSoIP service in its contact address AoC, the gateway IP-SM-GW then notifies the HLR (step H6) so as to tell it that all of the SMS messages destined for the terminal UE with the MSISDN number corresponding to the AoR (‘Address of Record’) field have to be routed to this gateway IP-SM-GW and no longer to the center MSC-C to which the terminal UE is tethered in parallel.
If the terminal UE started up on 4G, the terminal implements the reregistration procedure every 50 minutes, and upon each subsequent registration, the S-SCSF informs the gateway IP-SM-GW of the fact that the terminal is still registered.
The HLR is not updated by the gateway IP-SM-GW upon reception of the Third Party Registration frames extending these subsequent REGISTER.
It is assumed, with reference to FIG. 4B, that the terminal UE, which started up on 4G, carries out a handover to the 2G or 3G cellular network. Some terminal providers, on request from the telecommunications operators, may choose to maintain SIP registration under 2G and 3G cellular coverage, so as to optimize protocol exchanges within the telecommunications network and avoid ‘ping-pong’ effects. Other terminal providers may ask the terminals to deregister explicitly from the IMS network when the terminal is registered under 4G coverage and it performs a handover to 2G or 3G coverage. In this case, the terminal sends an explicit deregistration message to the IMS network (REGISTER with Expires=0). In any case, if the terminal has the 4G VxLTE service and the VxWiFi service, IMS registration is maintained during a 4G to WiFi handover, and vice versa, so as to offer clients service continuity between these two access networks.
If the terminal provider chooses to maintain IMS registration under 2G or 3G coverage, at the time of the following reregistration (step N2) upon expiry of the subsequent reregistration timer (50 minutes with Expires=1 hour), the terminal sends a subsequent REGISTER in which the SIP field PANI is positioned at GERAN or UTRAN, depending on whether the handover has been carried out to a 2G or 3G cellular network.
The gateway IP-SM-GW does not pass on any message to the HLR upon reception of the Third Party Registration frame (step N4).
The mechanism is similar in the case of a handover to a WiFi network, except that the terminal UE does not wait 50 minutes to carry out the subsequent registration (step N6), the field PANI changed to the value WLAN having to be passed on immediately in order for incoming telephone calls to be routed correctly.
The gateway IP-SM-GW does not send any message to the HLR upon reception of the Third Party Registration frame (step N8).
With reference to FIG. 4C, the terminal initially registered on 4G and having undergone a handover to a 2G cellular network deregisters by sending a REGISTER with a value EXPIRES equal to 0 (step N10). This mechanism is the same regardless of the type of connectivity at the time of the deregistration and regardless of the reason for the deregistration, in particular in the case of voluntary deregistration (for example when the user puts his terminal into ‘flight mode’ or shuts down his terminal via the ON/OFF button) or of deregistration on the initiative of the network.
Upon reception of such a deregistration message, the gateway IP-SM-GW tells the HLR (step J2) to no longer route the incoming SMS messages to the gateway IP-SM-GW, but to the last V-MCS used.
In summary, upon losing WiFi coverage, the terminal UE does not have time to deregister using SIP protocol, even though the gateway IP-SM-GW is belatedly informed of the deregistration of the terminal UE, more precisely upon expiry of the current registration (1 hour maximum) when the S-CSCF sends a message SIP DEREGISTER to the gateway IP-SM-GW (step N10) or via a message NOTIFY RegEvent if the gateway IP-SM-GW has subscribed to the event RegEvent in step G8.
As a result, if an SMS message is destined for the terminal UE, it is systematically routed, throughout this entire period of ambiguity, from the SMS-C to the gateway IP-SM-GW and then from this gateway to the S-CSCF, and then to the recipient terminal UE via the P-CSCF.
However, in this case, as the terminal UE is no longer able to be reached in packet-switched mode PS but only in circuit-switched mode CS, an error code is returned to the gateway IP-SM-GW, either by the S-CSCF or by the P-CSCF.
The gateway IP-SM-GW then uses the mode CS-Retry to interrogate the HLR via the MAP protocol to look for the address of the V-MSC to which the terminal UE is attached (in normal mode for 2G/3G network coverage or in Combined Attach mode for 4G coverage) in order to route the SMS message to this device.
The network resources are therefore not used optimally.
The invention targets a method for managing SMS messages that at least partly rectifies these problems.